Question: The Asian mystery bird in this striking photograph has very interesting eyes. Can you tell me about eye colour in this group of birds and how it relates to behaviour?

Response: This is an Asian barred owlet, Taenioglaux cuculoides, a small owl with a very large range. This species can be found from northeast Pakistan eastwards along the Himalayas to eastern China and south to the north of Malaysia. It is found in open forest and scrub from the lowlands up to about 2,800 metres. Unlike most pygmy owl species, which are nocturnal, the Asian barred owlet hunts large insects and other small prey mainly during the day.

The coloured portion of a bird's eyes is known as the iris -- the same name as for the coloured portion of human eyes. Owls' iris colour supposedly indicates the times when they are most active. Dark eyed owls are supposedly nocturnal (active during the night) whilst orange-eyed owls supposedly are crepuscular (active at dawn/dusk) or diurnal (active in the daytime) species. Yellow-eyed owl species are rumoured to be diurnal, as palfreyman mentioned, but in reality, owl species with yellow irises, such as this barred owlet, can be active at a variety of times. That said, the Asian barred owlet is a diurnal species. So really, iris colour doesn't correspond very well to when an owl species is most active, even though this "factoid" makes a fun story to tell people.

Another interesting feature of owls' eyes is the pupils can dilate and contract independently -- something that indicates a serious head injury if it happens in a human. But in owls, this is important: when sitting in the sun, the pupil of the eye in the sunlight will be very small, whilst the pupil of the eye in the shade will be very large. This is not so easy to see in a dark-eyed owl, but is quite obvious in a yellow-eyed species.

One more interesting feature of owl eyes is an anatomical feature they share with all birds -- they have a transparent (or translucent) "third eyelid". Known as the nictitating membrane, this thin membrane lies closest to the eye underneath the eyelid itself. The function of this membrane is to protect the eyes from damage.